The following relates to the nuclear power reactor arts, nuclear fuel assembly manufacturing and deployment arts, and related arts.
In nuclear reactor designs of the integral pressurized water reactor (integral PWR) type, a nuclear reactor core is immersed in primary coolant water at or near the bottom of a pressure vessel. In a typical design, the primary coolant is maintained in a subcooled liquid phase in a cylindrical pressure vessel that is mounted generally upright (that is, with its cylinder axis oriented vertically). A hollow cylindrical central riser is disposed concentrically inside the pressure vessel. Primary coolant flows upward through the reactor core where it is heated and rises through the central riser, discharges from the top of the central riser and reverses direction to flow downward back toward the reactor core through a downcomer annulus defined between the pressure vessel and the central riser. In the integral PWR design, at least one steam generator is located inside the pressure vessel, typically in the downcomer annulus. Some illustrative integral PWR designs are described in Thorne et al., “Integral Helical Coil Pressurized Water Nuclear Reactor”, U.S. Pub. No. 2010/0316181 A1 published Dec. 16, 2010 which is incorporated herein by reference in its entirety. Other light water nuclear reactor designs such as PWR designs with external steam generators, boiling water reactors (BWRs) or so forth, vary the arrangement of the steam generator and other components, but usually locate the radioactive core at or near the bottom of a cylindrical pressure vessel in order to reduce the likelihood of air exposure of the reactor core in a loss of coolant accident (LOCA).
The nuclear reactor core is built up from multiple fuel assemblies. Each fuel assembly includes a number of fuel rods. Spaced vertically along the length of the fuel assembly are grid assemblies which provide structural support to the fuel rods. At the top and bottom of the fuel assembly are an upper end fitting and a lower end fitting, respectively, providing structural support. The lower end fitting, sometimes called a nozzle plate, may be supported by a lower core support plate, support pedestals, or the like.
The lower end fitting is the entrance for coolant flow into its fuel assembly. The fuel assembly also includes guide tubes interspersed amongst the fuel rods. Control rods comprising neutron absorbing material are inserted into and lifted out of the guide tubes of the fuel assembly to control core reactivity. The guide tubes in a conventional assembly are rigidly attached to the grid assemblies, and the upper and lower end fittings are secured to ends of the guide tubes using fasteners to form the structural framework of the fuel assembly.
A dashpot is optionally disposed at the lower end of the guide tube to slow the descent of the control rod during a scram. The dashpot may be formed as a narrowed-diameter lower portion of the guide tube, or as a small-diameter dashpot tube disposed coaxially inside the lower end of the guide tube. A fastening arrangement, typically including a threaded fastener, connects the lower end fitting to the guide tube. The fastening arrangement optionally also includes a locking element to prevent the threaded connection from working its way out over time. Some illustrative fastening arrangements of this type are described, for example, in Walton, U.S. Pat. No. 4,036,692 which is incorporated herein by reference in its entirety, and in John Jr. et al., U.S. Pat. No. 5,068,083 which is incorporated herein by reference in its entirety.